1. Field of the Invention
The present invention relates to a method and apparatus for applying screw threads to a screw blank, the threads having a unique orientation. More specifically, the invention is directed to a device and method of cutting threads into a screw wherein the threads increase in cutting angle from the point to the top of the screw body. Furthermore, the screw core increases in diameter over the same length.
2. Description of the Prior Art
Screws have been utilized for a considerable period of time for the fastening of various objects to each other. Screws are particularly utilized for creating a joinder between two materials through the use of cutting threads which securely mount the shaft of the screw into a mounting substrate. Traditional screw threads cut a helical path through the substrate in an even and parallel form in order to maximize the binding characteristic of the screw while creating as little damage as possible to the substrate when the screw is inserted. While most screws are regular in the orientation of their threads and the sizing thereof, several screws have been developed which enjoy unique and irregular screw thread and core patterns.
A number of screws have been proposed which utilize a tapered central core in which the core of the screw and possibly the threads attached thereto has a smaller overall diameter at the point of the screw than at the head of the screw. This cone-shaped object is utilized for piercing and self-threading applications in which the screw is driven directly into a substrate without the need for a pilot hole or other starting techniques.
One shortcoming of screws having equally spaced and parallel threads is that the screw develops no compressive force on the substrate. While this is generally beneficial when utilized with a fragile substrate or an application in which the members to be joined are correctly oriented prior to insertion of the screw, in some applications it would be advantageous to develop a compressive force on the substrate exerted by the screw. This has been achieved in Lasner, a co-pending application, entitled "BONE SCREW WITH IMPROVED THREADS", bearing Ser. No. 07/618,500, filed Nov. 27, 1990, now U.S. Pat. No. 5,120,171. In this reference, a screw having threads which are progressively canted over the length of the screw is disclosed. The screw utilizes a tapered central core which is narrower at the point end and wider at the head end. The screw threads are angled at the point end in a fashion roughly perpendicular to the longitudinal axis of the screw core. As the threads progress along the screw core from the point end to the head end, the angle of the threads gradually increases from the nearly perpendicular with respect to the core surface to form a series of increasing obtuse angles with respect thereto. This screw develops a compressive force on the substrate material as it is inserted as the increasing cant of the threads over the length of the screw develops an increasing force on the substrate as they enter into it. An increasing continuum of compressive force is therefore developed along the longitudinal axis of the screw with the least amount of compressive force being present at the point end and the most compressive force being present at the head end. The reference does not, however, describe a method or apparatus for creating such threads on a traditional screw blank.
Screws may generally be manufactured in one of two ways. The first is to create a mold and cast a screw in the shape of the mold. The second is to begin with a screw blank and cut the threads into the blank utilizing a lathe or similar device. Even with the cast embodiment, an original prototype must be utilized to create the mold and no method or apparatus has been previously described which is capable of creating progressively canted threads along a screw body. Furthermore, while cutting devices have been proposed which permit the cutting of a tapered screw core with or without tapered threads thereon, no method or apparatus has been proposed which achieves the manufacturer of a screw having a tapered core and threads of varying cant.
What is lacking in the art, therefore, is a cutting device which is capable of variably cutting screw threads into a screw blank and which may further apply such threads to a screw blank having a tapered core.